This invention relates generally to the bromination of organic compounds and more particularly to an improved process for the ring bromination of activated aromatic compounds under mild conditions (no Br.sub.2) using a sulfoxide/HBr brominating agent such as dimethyl sulfoxide/HBr.
The one-step bromination of oxidatively unstable benzene derivatives, such as aniline and phenol, using a dimethyl sulfoxide/HBr brominating agent has been described in the literature. For example, see S. K. Srivastava et al., J. Chem. Soc. Chem. Comm., 1996, 2679-2680 and Majetich et al., J. Org. Chem. 1997, 62, 4321-4326. A large excess of brominating agent is used and especially the dimethyl sulfoxide which is also employed as a solvent for the reaction. Although suitable for academic studies, the use of excess brominating agent would be uneconomical for commercial purposes as this would require expensive product purification processes to remove and recover the excess for re-use. However, when an attempt was made to modify the processes of the literature so as to employ stoichiometric quantities of brominating agent, the conversion and product yields were significantly reduced. It has now been found that by removing the water from the reaction, very high conversions and yields (&gt;95%) can be achieved when using substantially stoichiometric quantities of reactants. The water is a by-product of the reaction and is also present when aqueous HBr is used.
In accordance with this invention, there is provided a bromination process which comprises reacting an activated aromatic compound with HBr and a sulfoxide, while removing water from the reaction mixture, so as to ring brominate the activated aromatic compound.